Over the last few decades, researchers have made significant developments in producing more advanced electrocatalytic materials for power generation applications. For example, traditional fuel cell catalysts often involve high priced precious metals such as Pt. However, in order for fuel cells to become commercially viable, there is a need to reduce or completely remove precious metal altogether. As a result, a myriad of novel, unconventional materials have been explored such as chalcogenides, porphyrins, and organic-metal-macrocycles for low/medium temperature fuel cells as well as enzymatic and microbial fuel cells.
Grain boundary (GB) phase transitions can change drastically the properties of polycrystals. The GB wetting phase transition can occur in the two-phase area of the bulk phase diagram where the liquid (L) and solid (S) phases are in equlibrium. Above the temperature of the GB wetting phase transition a GB cannot exist in equlibrium contact with the liquid phase. The experimental data on GB wetting phase transitions in numerous systems are analysed. The GB wetting tie-line can continue in the one-phase area of the bulk phase diagram as a GB solidus line. This line represents the GB premelting or prewetting phase transitions. The GB properties change drastically when GB solidus line is crossed by a change in the temperature or concentration. The experimental data on GB segregation, energy, mobility and diffusivity obtained in various systems both in polycrystals and bicrystals are analysed. In case if two solid phases are in equilibrium, the GB “solid state wetting” can occur. In this case the layer of the solid phase 2 has to substitute GBs in the solid phase 1. Such GB phase transition occurs if the energy of two interphase boundaries is lower than the GB energy in the phase 1.
MDF materials are chemically bonded ceramic materials free of the macrodefects typical of hydraulic cement-based materials. MDF materials arising through reactions of sulfo-aluminate-ferrite belitic (SAFB) clinkers and/or Portland cements (PC) with two types of water-soluble polymer (hydroxy-propylmethyl cellulose {HPMC}, polyphosphate glass {poly-P}) are discussed. Mixes of low energy SAFB clinkers with Portland cement, HPMC and, especially poly-P comprise promising cross-linked compositions additional to the better known MDF materials formed from high alumina cement with polyvinylalcohol/acetate. The principles of co-ordination of P and C atoms (of the polymer) with Al and Fe atoms (originating from the cement) are highlighted from spectroscopic information on next-nearest-neighbour interactions, along with the effects of second co-ordination spheres. Polymers modify the interface through functional bonding/grafting of polymer chains onto the surfaces of cement grains. Both the cross-linked atomic structure and the interface coincide well with the model of functional polymers and represent a new type of atomic-level structure in polymer-modified cements. Interpretation is based on previous magnetic resonance and thermal analysis studies. The compactness of Al(Fe)-O-P cross-links reduces transport through the interfaces, increasing the interfacial interactions and resisting the unfavourable uptake of moisture and carbonation.
The microscopic properties a ZnO grain boundary containing extrinsic point defects are studied using a density functional computational approach. The results show that the grain boundary acts as a sink for native defects, such as the zinc vacancy and the oxygen interstitial, and also for bismuth substitutional impurities. The defects tend to accumulate at under-coordinated sites in the boundary core and prefer to form small clusters. In particular the segregation of Bi promotes the formation of the other native defects by lowering their formation energies in the boundary. Individually, the native defects and the Bi impurity do not produce deep interface states in the band gap which are electrically active. However, when the defects cluster to form a BiZn-VZn-Oi complex, new gap states are created of acceptor type. It is suggested that these new states are caused by defect interactions which compensate one another resulting in the depletion of an occupied impurity state and new bond formation. The results are discussed in terms of the Schottky barrier model commonly used to describe the electrical characteristics of ZnO varistors.
The interfacial transition zone (ITZ) between aggregates and cement paste in cementitious materials is a crucial element in mechanical and transport systems. Computer simulation by the SPACE system is used to approach this problem in the present paper. For the particle-packing phenomenon in the fresh state of concrete, the SPACE system relies on a dynamic generation algorithm, reflecting the production conditions of concrete. Hence, structure of the model cement has been proven more realistic than can be achieved by random generator-based system.
The interfacial defect content of lamellar interfaces in Ti aluminide alloys has been evaluated by HREM and the data analysed using the Topological Theory of Interfacial Defects. It has been found that the defects observed are all perfect interfacial disconnections, and that the lamellar decomposition is diffusion-controlled. An analysis of the diffusive fluxes required for disconnection motion has been used to reconcile the apparent discrepancy between this conclusion and the martensitic crystallography exhibited by the TiAl lamellae. Moreover, this analysis has been used to explain why disconnections with Burgers vectors b = 1/3〈211〉 have been observed in these interfaces.
The intrinsic structure of different tilt grain boundaries in bcc molybdenum is determined by electron microscopy and compared to the ones obtained after an annealing treatment of the same boundaries in presence of different impurities like carbon and nickel. Specially grown bicrystals with tilt axes parallel to [001] and [011] are used. The boundaries correspond to the major coincidence relationships ∑ = 5, ∑ = 3 and ∑ = 11. Their experimental atomic structure is compared to calculated ones. After the treatments in presence of carbon or nickel the new structure is determined by electron microscopy from the structural and chemical aspect. After a treatment in presence of carbon the ∑ = 5[001]{310} boundary contains either a segregation or a very thin precipitate layer of a new MoCx quadratic phase. In presence of nickel, the physical phenomenon is possibly a wetting of the boundary. The different [011] tilt boundaries have a different behavior according to their respective energy.
Metal-oxide interfaces play an important role in spintronics—a new area of microelectronics that exploits spin of electrons in addition to the traditional charge degree of freedom to enhance the performance of existing semiconductor devices. Magnetic tunnel junctions (MTJs) consisting of spin-polarized ferromagnetic electrodes sandwiching an insulating barrier are such promising candidates of spintronic devices. The paper reviews recent results of first-principle density-functional studies of the atomic and electronic structure of metal-oxide interfaces in Co/Al2O3/Co and Co/SrTiO3/Co MTJs. The most stable interface structures, O-terminated for fcc Co (111)/α-alumina(0001) and TiO2-terminated with oxygens on top of Co atoms for fcc Co (001)/SrTiO3(001) were identified based on energetics of metal-oxide cohesion at the interface. The covalent character of bonding for both the Co/alumina and Co/SrTiO3 interface structures has been determined based on the pattern of electron distribution across the interface. The Al-terminated Co/alumina interface that corresponds to an under-oxidized MTJ exhibits a metallic character of bonding. The unusual charge transfer process coupled with exchange interactions of electrons in Co results in quenching of surface magnetism at the interface and substantial reduction of work of separation. The electronic structure of the O-terminated Co/Al2O3/Co MTJ exhibits negative spin polarization at the Fermi energy within the first few monolayers of alumina but it eventually becomes positive for distances beyond 10 Å. The Co/SrTiO3/Co MTJ shows an exchange coupling between the interface Co and Ti atoms mediated by oxygen, which results in an antiparallely aligned induced magnetic moment on Ti atoms. This may lead to a negative spin polarization of tunneling across the SrTiO3 barrier from the Co electrode. The results illustrate the important fact that spin-polarized tunneling in magnetic tunnel junctions is not determined entirely by bulk density of states of ferromagnet electrodes, but is also very sensitive to the nature of the insulating tunneling barrier, as well as the atomic structure and bonding at the ferromagnet/insulator interface.
Growth of Ni on (001) Au has been widely studied. Furthermore it has been shown that a structural change occurs when an Au/Ni multilayer is heated up. This transformation can be described as an ordering which is unexpected from the equilibrium phase diagram. A high-resolution electron microscopy study of the transformation of a coherent Au/Ni multilayer is presented in this paper. The transformation has been characterized by in-situ HREM and by extensive image processings made on both simulated and experimental images.
Using an ab initio total energy approach, we study the electronic structure of metal/MgO(100) interfaces. By considering simple and transition metals, different adsorption sites and different interface separations, we analyze the influence of the character of metal and of the detailed interfacial atomic structure. We calculate the interface density of states, electron transfer, electric dipole, and the Schottky barrier height. We characterize three types of electronic states: states due to chemical bonding which appear at well defined energies, conventional metal-induced gap states associated to a smooth density of states in the MgO gap region, and metal band distortions due to polarization by the electrostatic field of the ionic substrate. We point out that, with respect to the extended Schottky limit, the interface formation yields an electric dipole mainly determined by the substrate characteristics. Indeed, the metal-dependent contributions (interfacial states and electron transfer) remain small with respect to the metal polarization induced by the substrate electrostatic field.
We report on the distribution of micro-alloying elements in a multi-component TiAl-based alloy. The specimen contains 3 at.% Nb, 1.5 at.% Cr, 0.5 at.% Mn, 0.6 at.% (W + Hf + Zr), and 0.2 at.% each of B, C, and O. The distributions of all micro-alloying elements with respect to the heterophase interface between α2 and γ lamellae are analyzed with a three-dimensional atom-probe (3DAP) microscope. All the elements partition except boron, which resides primarily in boride precipitates. Oxygen, C, Mn, and Cr partition to the α2-phase, whereas Nb and Zr partition to the γ-phase. Both W and Hf exhibit excess concentration values within ca. 7 nm of the lamellar interface in the α 2-phase, and their near interfacial excesses are 0.26 and 0.35 atoms nm−2, respectively.
Zero flux planes (ZFP's) are the locations in a diffusion couple where fluxes of individual components vanish. The conditions required for developing ZFP's between interdiffusing multicomponent alloys are now well understood through the works of Dayananda and Morral. In this paper we analyze the kinetics of multicomponent diffusion near ZFP's. In contrast to the usual mixing that occurs between end-member alloys in a diffusion couple, where the average component compositions are approached through “global” exchange of atoms, the presence of a stationary ZFP requires instead that mixing of the blocked component occurs through the release of a pair of coupled diffusion waves. In classical (thick) couples these waves spread symmetrically away from the Matano plane. The wave pair consists of a “depletion” wave that reduces the blocked component concentration in the component-rich alloy, and a conjugate “repletion” wave that increases the concentration in the adjacent component-poor alloy. Curiously, each of these waves establishes the average (equilibrium) concentration unilaterally on either side of the Matano plane. A stationary ZFP precludes net transport of one component across the Matano plane—a circumstance that has practical and theoretically interesting implications for the design of stable multicomponent films and coatings that must resist diffusive loss of a component.
This paper describes the so called interfacial transition zone—ITZ—in concrete. This is the region of the cement paste around the aggregate particles, which is perturbed by the presence of the aggregate. Its origin lies in the packing of the cement grains against the much larger aggregate, which leads to a local increase in porosity and predominance of smaller cement particles in this region. The ITZ is region of gradual transition and is highly heterogeneous, nevertheless the average microstructural features may be measured by analysis of a large numbers of backscattered electron images of polished concrete samples. Such measurements show that the higher porosity present initially is significantly diminished by the migration of ions during hydration.
High-resolution electron microscopy is used to study interfaces between solids with varying degree of atomic ordering. Applying a recently developed technique the structure of amorphous germanium near (111) oriented crystalline silicon is described by its two-dimensional distribution function ρ(x,y) of atoms, and properties of ρ(x,y) are extracted from experimental images. Using extensive image simulations it is further shown that the technique is suitable to measure composition profiles at coherent heterointerfaces.
The subject of intergranular brittle fracture of metallic materials is reviewed, using iron, steel, and nickel-base alloys as examples. The main focus is on the influence of solute elements that either strengthen or weaken the boundaries. Time-dependent brittle fracture caused by hydrogen or surface-adsorbed impurities at elevated temperatures is included.
Twin boundaries (TBs) in ZnO sintered with small additions of Ga 2 O 3 have been characterized with advanced methods of transmission electron microscopy (TEM). The TBs and accompanying inversion domain boundaries are on {011¯3} planes of ZnO. The Ga content of the TB corresponds to an effectively half occupied {011¯3} plane determined from compositional maps calculated from electron spectroscopic images using electron filtering TEM. The structure of the TBs were investigated by high-resolution TEM, and images of focus series were used to reconstruct the complex electron wave. Simulated electron waves based on structure models of the TB were quantitatively compared with the reconstructed wave to identify and to refine atom positions. The twins can be considered to be created by a mirror operation on a {011¯3} plane of ZnO, and two alternating closed-packed polyhedral clusters of oxygen ions can be identified as building units of the TB structure. Unit 1 is occupied with Zn 2+ by simply continuing ZnO 4 tetrahedra of the same type from both crystals to the TB. Using arguments of local charge balance unit 2 can only be occupied with the trivalent Ga 3+ ion. The Ga 3+ position was refined with high precision (±5 pm), and the resulting polyhedron is a GaO 5 square pyramid. The pyramids form densely occupied columns parallel to the twin axis [21¯1¯0]. The analysis of the TB structure yields a fractional occupancy of the boundary plane by Ga of 0.5, which is in good agreement with the result of the chemical composition measurement with energy filtered TEM.
Thermally activated grain boundary migration (GB) is important in grain growth and thermo-mechanical processing of materials, but very little is known about the atomic-scale mechanisms involved. The purpose of this paper is to identify atomic-scale GB migration mechanism and investigate their dependence on GB structure. High-angle tilt and general GBs are investigated at elevated temperatures by high-resolution transmission electron microscopy (HREM) in Au and Al bicrystalline thin films. Digital analysis of HREM video recordings is used to detect atomic-scale structural changes at migrating GBs. GB motion typically is not smooth, but involves sharply varying speeds and thermally activated spatial fluctuations. Collective effects in GB migration are shown to exist and several different migration mechanisms are identified. Atomic-scale GB migration is found to depend on the macroscopic GB geometry as well as details of the interatomic interactions.
The paper reports on a two-stage study of the interface between three types of model cylindrical aggregates (sandstone, limestone and granite) and two types of mortar matrix (plain and 20% Silica Fume mortar). In the first stage, the surface roughness ( R a ) of the aggregates and the interfacial bond strength using push-out specimens have been experimentally determined. In the second, aggregate push-out geometry has been modelled using two different approaches. In the first approach, the surface roughness is ignored and the cylindrical aggregates are assumed to have an ideally smooth surface with a constant radius, r 0 over the aggregate length, L . In the second approach, the surface roughness of the aggregates is included so that the radius, r varies along the length of the cored rock aggregate. Hence, the influence of the surface roughness of the aggregates on the interfacial bond strength is obtained. It is found that the surface roughness plays a significant role in determining the interfacial bond strength, in particular of smaller size aggregates. The effect, however, diminishes as the aggregate size increases, regardless of the aggregate and mortar type.
The equilibrium crystal shape (ECS) of copper has been studied by scanning electron microscopy on μm-sized copper crystallites supported on single-crystals of α-alumina. In addition, the orientation relationships between copper crystals and the sapphire substrate were investigated by X-ray techniques. A detailed discussion of the kinetic factors that can inhibit equilibration is provided, and it is shown that only crystals ranging in radius from 3 to 4.5 μm can achieve equilibrium shapes under the conditions of the experiment. The maximum anisotropy of surface energy was found to be about 1.02, which is significantly lower than that of the other two fcc metals (lead and gold) for which reliable data are available. Another distinction between copper and those other fcc metals is that its ECS displays {110} facets, and possibly {311} facets, in addition to the commonly observed {111} and {100} facets, at temperatures where equilibration is possible. The observed facets connect tangentially to the curved parts of the ECS, so that all possible surface orientations are present on the copper ECS.
Sessile drop experiments of Ni and Ni(2at.%Al) were conducted under controlled working conditions, at 1500°C, P(O2) ≤ 10−9 Torr. It is shown that Al and oxygen atoms engaged in the capillary driven mass transport at the interface have a significant impact on the surface/interface thermodynamics. The surface energy of liquid Ni determined from experiments in which Ni comes into contact with Al2O3 is significantly lower than that of high purity Ni, due to the segregation of Al. The free energy of segregation of Al to the free surface of Ni (Δ GS) was found to range from −164 to −152 kJ/mol, indicating a relatively strong tendency for segregation of Al to the free surface of Ni(Al). It is proposed that an Al(O)-rich liquid layer forms adjacent to the Ni-Al2O3 interface, which improves interfacial adhesion. In the Ni(Al)-Al2O3 system, an increase in the Al content of the alloy leads to the improvement of both wetting and adhesion of the alloy on the ceramic, correlating with the improvement in the interface strength after solidification.
